From the Book

Splicing

Fiber-optic cables might have to be spliced together for a number of
reasonsfor example, to realize a link of a particular length. Another
reason might involve backhoe fade, in which case a fiber-optic cable
might have been ripped apart due to trenching work. The network installer might
have in his inventory several fiber-optic cables, but none long enough to
satisfy the required link length. Situations such as this often arise because
cable manufacturers offer cables in limited lengthsusually 1 to 6 km. A
link of 10 km can be installed by splicing several fiber-optic cables together.
The installer can then satisfy the distance requirement and avoid buying a new
fiber-optic cable. Splices might be required at building entrances, wiring
closets, couplers, and literally any intermediate point between a transmitter
and receiver.

Connecting two fiber-optic cables requires precise alignment of the mated
fiber cores or spots in a single-mode fiber-optic cable. This is required so
that nearly all the light is coupled from one fiber-optic cable across a
junction to the other fiber-optic cable. Actual contact between the fiber-optic
cables is not even mandatory.

There are two principal types of splices: fusion and mechanical. Fusion
splices use an electric arc to weld two fiber-optic cables together. The process
of fusion splicing involves using localized heat to melt or fuse the ends of two
optical fibers together. The splicing process begins by preparing each fiber end
for fusion. Fusion splicing requires that all protective coatings
be removed from the ends of each fiber. The fiber is then cleaved using the
score-and-break method. The quality of each fiber end is inspected using a
microscope. In fusion splicing, splice loss is a direct function of the angles
and quality of the two fiber-end faces.

The basic fusion-splicing apparatus consists of two fixtures on which the
fibers are mounted with two electrodes. An inspection microscope assists in the
placement of the prepared fiber ends into a fusion-splicing apparatus. The
fibers are placed into the apparatus, aligned, and then fused together.
Initially, fusion splicing used nichrome wire as the heating element to melt
or fuse fibers together. New fusion-splicing techniques have replaced the
nichrome wire with carbon dioxide (CO2) lasers, electric arcs, or gas
flames to heat the fiber ends, causing them to fuse together. Arc fusion
splicers can splice single fibers or 12- and 24-fiber-count ribbon fibers at the
same time. The small size of the fusion splice and the development of automated
fusion-splicing machines have made electric arc fusion one of the most popular
splicing techniques in commercial applications. The splices offer
sophisticated, computer-controlled alignment of fiber-optic cables to achieve
losses as low as 0.02 dB.

Splices can also be used as optical attenuators if there is a need to
attenuate a high-powered signal. Splice losses of up to 10.0 dB can be
programmed and inserted into the cable if desired. This way, the splice can act
as an in-line attenuator with the characteristic nonreflectance of a fusion
splice. Typical fusion-splice losses can be estimated at 0.02 dB for loss-budget
calculation purposes. Mechanical splices are easily implemented in the field,
require little or no tooling, and offer losses of about 0.5 to 0.75 dB.